1. Field of the Invention
The present invention relates to a method for enhancing image quality scanned by using an inspection tool.
2. Description of Related Art
During the fabrication of semiconductor, pattern defects occur on the mask or wafer reducing the yield to a great degree. Defect inspection and review are widely used for yield management. High resolution, high throughput, and low radiation damage on specimen are requested for defect inspection systems.
Defect inspection systems frequently utilize charged particle beams. In such systems, a charged particle beam, such as an electron beam (e-beam), is irradiated on structures to be inspected. The interaction of the electron beam with the inspected surface generates secondary electron signals, which is collected as grayscale images, and which are compared to detect defect by observing the change in gray scale.
As the dimensions of structures to be inspected are continuously getting smaller and the resolution of the inspection systems are getting higher, enormous amount of data are generated. In addition, because the secondary electron signals may be strongly affected by extremely irregular noise, the e-beam inspection typically use multi-scans, i.e., repeated scans, for a same sample to obtain low noise image.
After multi-scans, all scanned grayscales of each pixel are averaged, and a scanned image with low noise is thus obtained.
Theoretically, the more times of the scan have been made, the better image quality would be; however, the e-beam inspection is a time-consuming process, and the increased complexity of structures and times of scan increase the cost of defect inspections, both in terms of expense and time.
The scan times may be reduced or limited to lower the cost, but image quality would be a critical concern. Deteriorated image quality may ignore small defect and increase the false rate on defect determination, causes alignment error in the following alignment process, and even result in the devices to fail. As the dimensions of semiconductor devices decrease, inspection becomes even more important and image quality should be promoted.
Median filter is a nonlinear digital filtering technique used to remove noise. The main idea of the median filter is to run through the signal entry by entry, replacing each entry with the median of neighboring entries. In one image frame, a pixel with its neighbor can form a matrix, and a median value can be determined from this matrix to replace the possible extreme value of this pixel.
The median filter is not suitable for the defect inspection systems. If a defect happens to one pixel only, this defect will be ignored by the median filter. In addition, larger image distortion occurs when tuning brightness and contrast.
In short, scan times should be reasonably reduced for saving time and cost. In limited scan times, the image quality has to be promoted, but qualified images are not always provided through a traditional averaging or median filter method. Therefore, it would be advantageous to provide methods or systems for enhancing the image quality.